A spark plug used in an internal combustion engine has an insulator that includes, for example, an alumina-based sintered body containing alumina as a main component. An alumina-based sintered body is generally formed by sintering a powder mixture containing sintering aids including SiO2, MgO, and the like (for example, Japanese Unexamined Patent Application Publication No. 2014-44892). In such an alumina-based sintered body, the sintering aids are mainly contained in glass that is present at alumina crystal grain boundaries. Such glass is present as a low melting point glass phase. In consideration of a problem that the low melting point glass phase is softened in an environment at about 700° C. and the withstand voltage performance of the insulator decreases, Japanese Unexamined Patent Application Publication No. 2014-44892 specifies appropriate insulator materials.